As one kind of a capacitor, there is a film capacitor having a structure such that, while using a flexible resin film as a dielectric, first and second counter electrodes that oppose each other with the resin film interposed therebetween are disposed. A film capacitor typically has a form of an approximately cylindrical shape obtained by winding the above-described resin film serving as the dielectric. First and second external terminal electrodes are formed respectively on first and second end surfaces of the cylinder that oppose each other. The first counter electrode is electrically connected to the first external terminal electrode, and the second counter electrode is electrically connected to the second external terminal electrode.
As a dielectric resin film for the film capacitor, Japanese Patent Application Laid-open (JP-A) No. 10-119127 (Patent Document 1), for example, proposes a biaxially oriented polypropylene film and also proposes a film capacitor using this biaxially oriented polypropylene film as a dielectric resin film and being excellent in heat resistance and insulation breakdown characteristics. Polypropylene which is used as a material for a dielectric resin film of a film capacitor is a thermoplastic resin, and the upper limit of the temperature for the use thereof is typically 85° C.; however, in the polypropylene disclosed in Patent Document 1, the dielectric breakdown strength at 105° C. is improved to be 540 V/μm, thereby enabling use at a higher temperature.
However, as described above, in the dielectric resin film disclosed in Patent Document 1, the heat resistance is not yet satisfactory because the ensured temperature, though improved, is up to 105° C. For example, the dielectric breakdown strength at 125° C. is not disclosed in Patent Document 1. For this reason, it is not clear whether the dielectric resin film can be used for purposes that require high voltage resistance under a higher-temperature environment.
On the other hand, International Publication No. 2010/101170 Pamphlet (Patent Document 2) discloses the use of a dielectric resin film made of a thermosetting resin. A dielectric resin film made of a thermosetting resin has characteristic features such that the heat resistance is high; the dielectric breakdown strength of the resin is inherently high; and, in the case of a resin in which the glass transition temperature is above or equal to the ensured temperature of 125° C., the dielectric breakdown strength at a high temperature is high.
In particular, as a resin excellent in voltage resistance, a resin obtained by curing a mixture liquid of polyvinylacetoacetal (PVAA) and tolylenediisocyanate (TDI) disclosed in Patent Document 2 has an extremely high dielectric breakdown strength and can be used for purposes that require high voltage resistance.
On the other hand, in accordance with an increasing frequency of electronic apparatus in recent years, it is demanded that the characteristics of electronic components at a high frequency are good. A major problem in capacitors is the dielectric loss, that is, tan δ (dielectric tangent), at a high frequency. The dielectric loss tan δ is preferably as low as possible and is desirably zero. When this value is high, energy loss and heat generation accompanying this are generated, thereby inviting problems such as unstable operation and short lifetime of a high-frequency circuit. In the PVAA/TDI, the dielectric loss tan δ is as high as 0.75% at 25° C., 1 kHz, thereby raising a problem in that the use is difficult for purposes in which the heat generation is a problem.
As will be understood from the above description, a material having a high dielectric breakdown strength at a high temperature and having a tan δ of 0.5% or less has not been realized so far.
Next, Japanese Patent Application Laid-open (JP-A) No. 2010-87507 (Patent Document 3) proposes a film capacitor having two dielectric layers, wherein a first dielectric layer includes polyethylene terephthalate or polypropylene, and a coating layer made of a mixture of a thermoplastic resin and a thermosetting resin is disposed as a second dielectric layer immediately above the first dielectric layer.
However, according to the structure disclosed in Patent Document 3, the heat resistance and the electrical characteristics are dominated by the dielectric layer having the poorer performance of the two dielectric layers. Therefore, the technique disclosed in Patent Document 3 is not satisfactory for improving the heat resistance and the electrical characteristics.
Patent Document 1: Japanese Patent Application Laid-open (JP-A) No. 10-119127
Patent Document 2: International Publication No. 2010/101170 Pamphlet
Patent Document 3: Japanese Patent Application Laid-open (JP-A) No. 2010-87507